These contrails have a surprisingly big but also complex effect on the climate.

Because they are clouds, they trap heat that is emitted by the Earth's surface, creating a "greenhouse effect" that adds to warming.

Yet during daytime, these clouds have a cooling effect because they are white and thus reflect some of the Sun's energy back into space. In certain conditions, contrails can exist for several hours.

Stuber and team estimate the radiation caused by contrails at a busy flight corridor in southeast England.

Using high-resolution aircraft flight data and routine weather balloon data, they looked at "persistent" contrails: wakes that remained for an hour or more after the aircraft had flown over.

Night flights account for only 22% of Britain's annual air traffic but contribute between 60 to 80% of the greenhouse effect from contrails, the scientists found.

Winter flights warm more

Stuber and team also found that flights during the winter months could contribute more to global warming.

"We also found that flights between December and February contribute half of the annual mean climate warming, even though they account for less than a quarter of annual air traffic," says Stuber.

Although there are fewer flights during the winter months, the conditions needed to form contrails - the right temperature, amount of moisture in the air and aircraft altitude - are found more often then.

A growing problem

Global emissions of man-made CO2 are between 6.2 billion and 6.9 billion tonnes per year. Added to this are around 1.5 billion tonnes from land use.

Commercial aircraft account for only a small contribution compared with power stations, industry and road traffic.

However, passenger travel is growing at the rate of around 5% a year, which means that this share will grow fast.

A 1999 estimate by the UN's Intergovernmental Panel on Climate Change (IPCC) found that the airline industry accounted for 2% of man-made CO2 emissions in 1992. But it would rise to as much as 15% by 2050.

Environmentalists are angry, complaining that airlines get a free ride when it comes to environmental taxes.

Changing altitude could also help

In addition to rescheduling night flights for the daytime, planes could diminish their contribution to global warming by changing their altitude.

A study published last year in the journal Transportation Research suggests that the regions of "ice-supersaturated" air where contrails form is only about 500 metres thick.

The goal would be to fit sensors on aircraft that could inform pilots where this layer lies, thus enabling them to shift altitude accordingly.